Baths containing "free" cyanide as the alkali metal salt(s) and metal cyanide complexes are frequently used for electroplating copper, zinc, silver, gold, brass and other metals because the high "throwing" power of cyanide electrolytes promotes uniformity of deposits, because the rate of metal deposition is high for both large and small parts, and because of compatibility with various conventional additives which can be used to control the character of the plated surfaces. Some cyanide ions are oxidized to form carbonate ions, and this reaction appears to be accelerated by visible radiation, and by increased temperatures (H. Graham Silver, Jour. Electrochemical Soc., 116, 26C-32C, (1969)). The alkali metal hydroxides in the cyanide baths also react with carbon dioxide from the surrounding air to produce the alkali metal carbonate. Carbonates are reported as being formed by hydrolysis of cyanide.
A limited amount of carbonate salts can be desirable because of improved electrical conductivity, or to provide alkalinity for chemical buffering action in systems which contain hydroxide ions. Excessive amounts of carbonate salts decrease the cathode efficiency when brass or copper is being plated from cyanide-containing baths, and adversely affect the appearance of the plated surface. Usually, the carbonate salts of cyanide-containing plating baths are controlled by "drag out" of plating solution with plated products, by controlled blowdown of bath to waste, by crystallization of alkali metal carbonates by natural and artificial refrigeration, by precipitation as an alkaline earth carbonate (see U.S. Pat. No. 2,164,924), by the use of mineral and organic acids, and, more recently, by an ion exchange process (U.S. Pat. No. 3,661,734). In each of these processes, it is either necessary or desirable to start with a relatively concentrated solution of carbonate salt(s). For this reason, the concentration of carbonate salts, in plant operation, is usually above the desired or optimum value for good plating.
There are problems associated with the prior art processes. Artificial refrigeration is expensive. Chemicals are discarded, or are used to make salts which are discarded. Valuable metals and salts are lost unless the solids such as calcium carbonate and sodium carbonate are rinsed, and the rinse solution is recycled. Rinse solution requires evaporation to concentrate the plating bath components. If recovery is not practiced, oxidizing chemicals such as chlorine, hypochlorites, hydrogen peroxide, or permanganate are required to destroy the cyanide, and alkaline chemicals are required to precipitate the metals as hydroxides. The solid carbonates and precipitated metal hydroxides must ultimately be disposed of in a safe and suitable manner.